Friction stir welding using a superabrasive tool

ABSTRACT

A probe for friction stir welding MMCs, ferrous alloys, non-ferrous alloys, and superalloys, as well as non-ferrous alloys, the probe including a shank, a shoulder, and a pin disposed through the shoulder and into the shank, wherein the pin and the shoulder at least include a coating comprised of a superabrasive material, the pin and shoulder being designed to reduce stress risers, disposing a collar around a portion of the shoulder and the shank to thereby prevent movement of the shoulder relative to the shank, and incorporating thermal management by providing a thermal flow barrier between the shoulder and the shank, and between the collar and the tool.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This document claims priority to U.S. Provisional Patent SerialNo. 60/202,665, filed May 8, 2000 and entitled FRICTION STIR WELDINGUSING A SUPERABRASIVE TOOL.

BACKGROUND

[0002] 1. The Field of the Invention

[0003] This invention relates generally to friction stir welding whereinheat for creating a weld is generated by plunging a rotating pin of atool into a workpiece. More specifically, the present invention relatesto a new tool that is used in a friction stir welding process thatenables the present invention to weld materials that are notfunctionally weldable using state of the art friction stir weldingprocesses and tools, said materials including ferrous alloys such asstainless steel, and higher melting point super alloys that contain onlysmall amounts of or no ferrous materials at all.

[0004] 2. Background of the Invention

[0005] Friction welding has been used in industry for years. It is asolid-state process that yields large economic benefits because itavoids many problems associated with rapid solidification of moltenmaterial that occurs in traditional fusion welding processes.

[0006] One example of friction welding occurs when the ends of two pipesare pressed together while one pipe is rigidly held in place, and theother is pressed against it and turned. As heat is generated byfriction, the ends of the pipes become plasticized. By quickly stoppingrotation of the pipes, the two pipes fuse together. Note that in thiscase, the frictional heating is caused by the relative motion of the twoparts to be joined.

[0007] In contrast, FIG. 1 is a perspective view of a tool being usedfor friction stir butt welding that is characterized by a generallycylindrical tool 10 having a shoulder 12 and a pin 14 extending outwardfrom the shoulder. The pin 14 is rotated against a workpiece 16 untilsufficient heat is generated, wherein the pin of the tool is plungedinto the plasticized workpiece material. The workpiece 16 is often twosheets or plates of material that are butted together at a joint line18. The pin 14 is plunged into the workpiece 16 at the joint line 18.The frictional heat caused by rotational motion of the pin 14 againstthe workpiece material 16 causes the workpiece material to softenwithout reaching a melting point. The tool 10 is moved transverselyalong the joint line 18, thereby creating a weld as the plasticizedmaterial flows around the pin from a leading edge to a trailing edge.The result is a solid phase bond 20 at the joint line 18 that isgenerally indistinguishable from the workpiece material 16.

[0008] The prior art is replete with friction stir welding patents thatteach the benefits of using the technique to obtain welds that havebeneficial characteristics over contemporary fusion welding processes.These benefits include low distortion in long welds, no fumes, noporosity, no splatter, and excellent mechanical properties regardingtensile strength. Furthermore, the process has the advantage of using anon-consumable tool, no need for filler wire, no need for gas shielding,and a tolerance for imperfect weld preparations such as the presence ofoxide in the weld region. The process is especially useful forpreventing significant heat damage or otherwise altering the propertiesof the original material being welded.

[0009] However, it has long been a desire of industry to be able to weldmaterials that are presently functionally unweldable for friction stirwelding. Thus, while friction stir welding is a very advantageoustechnique for welding non-ferrous alloys such as aluminum, brass andbronze, there has been no tool that is capable of functionally weldingmaterials having higher melting points. It should be understood thatfunctionally weldable materials are those that are weldable usingfriction stir welding in more than nominal lengths, and withoutdestroying the tool.

[0010] Unfortunately, fusion welding alters or damages the alloy at theweld, thereby compromising the weld as a result of the defects oradverse phases which form in the weld during the welding process. Insome cases, the non-metallic reinforcement material which has beenjoined with the original workpiece material to create the alloy isdepleted at the weld. The result is a weld that has properties andcharacteristics which are different from the unaltered areas of theoriginal workpiece material.

[0011] Until now, it has been the nature of friction stir welding thatusing a conventional friction stir welding tool or probe is worn downsignificantly so as to prevent functional welding of materials such asMMCs, ferrous alloys, and superalloys. Most tools simply do not work atall in MMCs, ferrous alloys, and superalloys. If a conventional toolcould begin friction stir welding, the wear would be so significant thata probe would be torn apart after only a short distance. For example,some alloys will cause wear on a probe such that it can no longerfunction after welding for a distance of only inches.

[0012] Unfortunately, it is generally the case that it is not possibleto simply insert a new tool and begin the friction stir welding processwhere the previous probe failed. If the weld is not continuous anduninterrupted, it is useless because of mechanical weakness.Furthermore, a portion of the tool is typically left behind in theworkpiece material, also contributing to the mechanical weakness.

[0013] Therefore, it would be an advantage over the prior art to providea new tool for use with the friction stir welding process that enableslonger continuous and uninterrupted welding runs (functional welding) ofmaterials that will cause a conventional tool to fail after a shortdistance. It would also be an advantage over the prior art if the newtool made it possible to friction stir weld materials that werepreviously too difficult to weld with conventional friction stir weldingtools. It would also be an advantage to provide a tool that would enablefriction stir welding with conventional workpiece materials, whileexhibiting improved wear characteristics for the tool.

[0014] A first class of materials that would be desirable to frictionstir weld but are functionally unweldable with conventional tools areknown as metal matrix composites (MMCs). An MMC is a material having ametal phase and a ceramic phase. Examples of the ceramic phase includesilicon carbide and boron carbide. A common metal used in MMCs isaluminum.

[0015] MMCs have desirable stiffness and wear characteristics, but theyalso have a low fracture toughness, thereby limiting applications. Agood example of a use for MMCs is in disk brake rotors on vehicles,where stiffness, strength and wear provide advantages over presentmaterials, and where the more brittle nature is generally not an issue.The MMC makes the rotor lighter than cast-iron, and the ceramic phasesuch as silicon carbide enables greater wear resistance.

[0016] Other important applications for MMCs include, but should no beconsidered limited to, drive shafts, cylinder liners, engine connectingrods, aircraft landing gear, aircraft engine components, bicycle frames,golf clubs, radiation shielding components, satellites, and aeronauticalstructures.

[0017] A second class of materials that would be desirable to frictionstir weld, and which have much broader industrial applications, areferrous alloys. Ferrous alloys include steel and stainless steel.Possible applications are far-ranging, and include the shipbuilding,aerospace, railway, construction and transportation industries. Thestainless steel market alone is at least five times greater than themarket for aluminum alloys. It has been determined that steels andstainless steels represent more than 80% of welded products, making theability to friction stir weld highly desirable.

[0018] Finally, a third class of materials that would be desirable tofriction stir weld, have broad industrial applications, have a highermelting point than ferrous alloys, and either have a small amount ofiron or none, are the super alloys. Superalloys are nickel, iron-nickel,and cobalt-base alloys generally used at temperatures above 1000 degreesF. Additional elements commonly found in superalloys include, but arenot limited to, chromium, molybdenum, tungsten, aluminum, titanium,niobium, tantalum, and rhenium.

[0019] It is noted that titanium is also a desirable material tofriction stir weld. Titanium is a non-ferrous material but has a highermelting point than other non-ferrous materials.

[0020] There are significant challenges that have so far prevented thecreation of a tool that can functionally weld MMCs, ferrous alloys, andsuperalloys. Some of these challenges only became apparent duringexperimentation as the inventors initially attempted to modify existingtools that can friction stir weld non-ferrous alloys. These challengesand the evolution of the tool will be discussed so as to enable thereader to practice the invention.

SUMMARY OF INVENTION

[0021] It is an object of the present invention to provide a new toolfor use in friction stir welding that has improved wear characteristicsover conventional tools.

[0022] It is another object to provide the new tool that includes asuperabrasive material that enables friction stir welding of MMCs,ferrous alloys, and superalloys, as well as non-ferrous alloys.

[0023] It is another object to provide the new tool that enablesimproved weld characteristics for the non-ferrous alloys.

[0024] It is another object to provide the new tool that enablesfinishing costs of a welded material to be reduced.

[0025] It is another object to provide the new tool that has an improvedgeometry to reduce wear of the tool when friction stir welding MMCs,ferrous alloys and superalloys.

[0026] It is another object to reduce thermal, mechanical and chemicalwear of the new tool.

[0027] It is another object to provide thermal management for the newtool.

[0028] In a preferred embodiment, the present invention is a tool forfriction stir welding MMCs, ferrous alloys, non-ferrous alloys, andsuperalloys, the tool including a shank, a shoulder, and a pin disposedthrough the shoulder and into the shank, wherein the pin and theshoulder at least include a coating comprised of a superabrasivematerial, the pin and shoulder being designed to reduce stress risers,disposing a collar around a portion of the shoulder and the shank tothereby inhibit rotational movement of the shoulder relative to theshank, and incorporating thermal management by providing a thermal flowbarrier between the shoulder and the shank, and between the collar andthe tool.

[0029] In a first aspect of the invention, the shank, shoulder, and pinare separate components that are coupled together to form the frictionstir welding tool, wherein the shoulder and the shank include asuperabrasive coating.

[0030] In a second aspect of the invention, the shank and the shoulderare a monolithic element including a superabrasive coating over at leasta portion thereof, and having a separate pin with a superabrasivecoating.

[0031] In a third aspect of the invention, the shank, shoulder and pinare a monolithic element having a superabrasive coating covering atleast a portion thereof.

[0032] In a fourth aspect of the invention, thermal management of heatusing thermal flow barriers within the tool enables sufficient heat tobe generated at the pin to enable friction stir welding, whileprotecting a tool holder from heat damage.

[0033] In a fifth aspect of the invention, stress risers are reduced onthe pin, larger radii are provided on the shoulder, and pin diameter isincreased to thereby enable friction stir welding of MMCs, ferrousalloys, and superalloys.

[0034] In a sixth aspect of the invention, the tool includes at leastone CVD, ion-beam implanted, and/or PVD coating disposed over thesuperabrasive coating to thereby increase resistance to chemical andmechanical wear.

[0035] In a seventh aspect of the invention, the tool is coated with awhisker reinforced superabrasive in order to decrease spalling of thesuperabrasive coating.

[0036] In an eighth aspect of the invention, flats are disposed alongthe lengthwise axis of the tool to thereby prevent separation of thetool into component elements during translational motion of the tool.

[0037] In a ninth aspect of the invention, the superabrasive coating isselected based upon a desired balance between chemical wear andmechanical wear.

[0038] In a tenth aspect of the invention, the superabrasive coating isselected based upon the characteristic of having a low coefficient offriction that prevents the workpiece material from adhering to the tool,thereby reducing wear of the tool.

[0039] These and other objects, features, advantages and alternativeaspects of the present invention will become apparent to those skilledin the art from a consideration of the following detailed descriptiontaken in combination with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 is a perspective view of a state of the art friction stirwelding tool that is welding two plates of material together.

[0041]FIG. 2A is a cross-sectional profile view of the preferredembodiment, made in accordance with the principles of the presentinvention.

[0042]FIG. 2B is an end view of the tool shown in FIG. 2A.

[0043]FIG. 3 is a cross-sectional view of an alternative embodiment,where the shank, shoulder and pin are all separate components.

[0044]FIG. 4 is a cross-sectional view of another alternative toolembodiment, where a hole is disposed through the length of the shank toassist in pin replacement.

[0045]FIG. 5 is a cross-sectional view of another alternative toolembodiment, where the shank, shoulder and pin are a monolithic element.

[0046]FIG. 6A is a cross-sectional view of an endmill blank that isfunctioning as a pin, the pin having helical channels in which isdisposed superabrasive material.

[0047]FIG. 6B is an end view of the endmill blank of FIG. 6A.

[0048]FIG. 7A is a cross-sectional view of another alternative toolembodiment, where the shank also functions as a locking collar.

[0049]FIG. 7B is a close-up profile view of surface irregularities thatenables mechanical locking to thereby prevent slipping of the shoulderrelative to the shank.

[0050]FIG. 7C is a close-up profile view of surface irregularities thatenables mechanical locking to thereby prevent slipping of the shoulderrelative to the shank.

[0051]FIG. 8 is a cross-sectional profile view of a pin having surfacedeformations designed to create transitional or turbulent flow aroundthe pin in the workpiece material.

[0052]FIG. 9A is an end view of a pin that includes surface deformationsin the form of a flat on the pin designed to create transitional orturbulent flow around the pin.

[0053]FIG. 9B is an end view of a pin that includes surface deformationsin the form of an irregular surface on the pin designed to createtransitional or turbulent flow around the pin.

[0054]FIG. 10 is cross-sectional profile view of a tool that has an offcenter pin or cam designed to create transitional or turbulent flowaround the pin.

DETAILED DESCRIPTION

[0055] Reference will now be made to the drawings in which the variouselements of the present invention will be given numerical designationsand in which the invention will be discussed so as to enable one skilledin the art to make and use the invention. It is to be understood thatthe following description is only exemplary of the principles of thepresent invention, and should not be viewed as narrowing the claimswhich follow.

[0056] The presently preferred embodiment of the invention is a toolthat incorporates superabrasive materials in a pin and shoulder, andutilizes thermal management within the tool, to enable friction stirwelding of materials that are presently functionally unweldable. Thus,the present invention makes possible long, continuous, and uninterruptedwelds of MMCs, ferrous alloys, and superalloys without sufferingsignificant degradation of the tool.

[0057] The development of the presently preferred embodiment presentedsignificant challenges because conventional tools wore out or broke whenused on MMCs, ferrous alloys, and superalloys. These challenges can besummarized as thermal wear, mechanical wear, chemical wear, thermalmanagement and geometry of the tool. The solutions to these challengesposed significant problems until the materials selected for the toolwere combined with the correct tool geometry and thermal management, aswill be explained through illustration of various embodiments of theinvention.

[0058] The sequence of events that culminated in the tool of the presentinvention that is capable of functionally welding MMCs, ferrous alloys,and superalloys began with a test on a broken tool. This broken tool hada pin broken off while running a weld on an workpiece material made fromMMC. Therefore, only a raised shoulder formed of CBN was disposedthereon, with a locking collar. The inventors never expected theshoulder to withstand the chemical or mechanical wear on a ferrousmaterial, but wondered what would happen. Surprisingly, the shouldershowed no significant signs of wear after a long and continuous run overa workpiece having a high melting point.

[0059] The success of the test led the inventors to experiment withvarious tool embodiments, trying to identify those characteristics ofthe tool that could take advantage of the surprising wear and thermalresistance results of the broken tool.

[0060]FIG. 2A is a cross-sectional profile view of the elements of thepreferred embodiment of the present invention that is a result of thosetests. Beginning with the mechanical elements, the tool 28 includes ashank 30 that is generally cylindrical. Coupled to the shank 30 is ashoulder 32 with an integral pin 34. Coupled around a portion of theshank 30 and the shoulder 32 with an integral pin 34 is a collar 36.Disposed between the shank 30 and the shoulder 32 with an integral pin34 is a thermal flow barrier 38. There is also a thermal flow barrier 40disposed between the collar 36 and a portion of the shank 30, as well asthe shoulder 32 with an integral pin 34.

[0061] This preferred embodiment incorporates several novel elements,only some of which are readily apparent from FIG. 2A. First, thepreferred materials used in construction of the tool 30 are critical tothe invention. The shank 30 is preferably cemented tungsten carbide.Cemented tungsten carbide is selected for its strength, and for its highthermal conductivity that allows proper cooling of the shank to maintainits strength relative to the other materials used in the tool 28.

[0062] One advantageous characteristic of the superabrasive material isits high thermal conductivity. However, it is important to understandthat the thermal conductivity can be useful or a detriment to the tool,depending upon how it is managed. Experimentation has demonstrated thatthermal management is critical to creating a successful friction stirwelding tool.

[0063] For example, when the pin 34 is coated with a superabrasivematerial, this resulted in significant amounts of heat being drawn awayfrom the weld region. To compensate, the tool 28 had to be driven harderthan desired to create more heat in the weld region. Therefore, asuccessful tool has to direct sufficient heat to the weld region toenable solid-phase welding, while at the same time limiting the heat sothat the weld region is kept as cool as possible in order to obtain ahigh quality weld region. In other words, with high thermal conductivityof the superabrasive, the tool can be designed to regulate any desiredflow of heat out of the tool, thereby enabling design flexibility. Incontrast, a material with lower thermal conductivity would be limited toits own value of thermal conductivity, or less.

[0064] Where a lot of heat is generated by the tool because of requiredrun parameters, it may be necessary to resort to external cooling of thetool. The objective is to have a tool whose thermal flow characteristicscan be modified in order to obtain the best weld characteristics,including a weld that cools fast.

[0065] A thermal management scheme was developed in order to maintainthe heat generated by friction between the tool and the workpiece nearthe weld region. One aspect of the scheme is to select a material forthe shank 30 that will restrict heat flow from the pin 34, to a toolholder (not shown) that is gripping the attachment end 42 of the tool28. The tool holder causes the tool 28 to rotate, and it might also bedamaged by heat. The thermal management scheme also keeps the shank coolenough to resist translational forces during friction stir welding.

[0066] Alternatively, a high strength steel can be substituted for thecemented tungsten carbide in the shank 30, but the steel will conductless thermal energy away from the shoulder 32 and integral pin 34,thereby causing the shank to run at a higher temperature and reducedstrength. However, the steel will function with the proper cooling.

[0067] If rotating the pin 34 against the workpiece material is whatenables heat to be generated for the friction stir welding process, itis important to know what rates of rotation will result in a functionalweld. The rate of rotation of the shoulder 32 with an integral pin 34 ispreferably within the range of 50 rpm to 2000 rpm, depending upon thematerial being welded, the diameter of the tool, and the composition ofthe elements of the tool 28. It is noted that the preferred surfacespeed of the tool is between 7 and 400 surface feet per minute.

[0068] Along with the rate of rotation, the timing of the friction stirwelding process is not trivial either. It is important that the pin 34be plunged into the workpiece material only when it is sufficientlyheated to form a plasticized welding region. However, this timingchanges for the materials being used, for each tool configuration, andfor the process parameters used.

[0069] The purpose of the thermal flow barrier 38 can now be understoodin light of the previous comments regarding thermal flow management. Itis critical that the frictional heat be properly managed to keep heatfocused on the workpiece material without drawing it away through thetool 28. In the presently preferred embodiment, titanium or a titaniumalloy is selected as the material for the thermal flow barrier 38. Atitanium alloy is selected because of its ability to withstand thetemperatures that are experienced by the tool 28, and because of itsrelatively low thermal conductivity. Nevertheless, it should be realizedthat the titanium alloy is not the only material that can be used. It isdescribed for illustration purposes, and can be replaced with a materialthat performs a similar function.

[0070] The shoulder 32 with integral pin 34 is a most novel element ofthe invention because of the materials used in its fabrication, andbecause of its geometry. These elements are selected in order toovercome the extreme thermal, mechanical, and chemical wear of thefriction stir welding process. One type of wear is not necessarily moreimportant than another, they just result in different types of failures.

[0071] Regarding material, it has been determined throughexperimentation that using a superabrasive on the shoulder 32 and thepin 34 has enabled the invention to achieve functional welding of MMCs,ferrous alloys, non-ferrous alloys, and superalloys. Specifically in thepreferred embodiment, polycrystalline cubic boron nitride (PCBN) is usedas a superabrasive-coating on a substrate material being used for theshoulder 32 with the integral pin 34.

[0072] To state that a coating of PCBN is utilized as a superabrasivecoating on the substrate material might imply that the applicationprocess is trivial. This is far from the case. The coating is not merelya substance that is wiped on using a room temperature process. Rather,the application involves high temperatures and ultra high pressures.Furthermore, the geometry of the surface to which the superabrasive isapplied has much to do with the ability of the superabrasive to wearwell, and to avoid failure from cracking. Accordingly, an importantaspect of the invention is to describe a tool geometry that will obtainthe best results from a superabrasive coating. Another important aspectof the invention is to describe the possible superabrasive materialsthat can be used. One example of how the coating is applied will now beprovided.

[0073] PCBN is made from hexagonal boron nitride power in an ultra hightemperature and ultra high pressure (UHTP) press (one million psi at1400 degrees Celsius, or 1673 K). Time and temperature are adjustable tocreate cubic boron nitride crystals having the optimum size, shape andfriability for specific applications. The crystals range in size ofdiameter from under a micron to around 50 microns.

[0074] For fabricating the shoulder 32 and integral pin 34 of thepresent invention, the cubic boron nitride (CBN) crystals are mixed witha powder of a different or second phase material. The second phasematerial is either ceramic or metal based. The CBN provides mechanicalstrength, while a ceramic would provide resistance to chemical wear.Therefore, the percentage of CBN relative to the second phase materialis dependent upon the application, where a balance must be struckbetween mechanical and chemical wear resistance.

[0075] It has been determined that the second phase material generallyadds a toughness and chemical stability to the PCBN. The toughness is inpart due to the ability of the second phase to inhibit crackpropagation. The CBN helps here as well, as it has randomly orientedfracture plans that naturally resist spalling. Lower CBN content isgenerally used for machining operations of hardened high temperaturesuperalloys needing more chemical wear resistance and less mechanicalwear resistance. Higher CBN content is used for abrasive wearresistance, where the second phase is generally metallic for addedtoughness.

[0076] It is important to note that CBN crystals have hardness values,thermal conductivity, thermal expansion, coefficient of friction values,fracture toughness, and transverse rupture values similar to diamond.These properties are engineered using the second phase material toachieve a specific application requirement.

[0077] The mixed powder is placed with a substrate such as cementedtungsten carbide, or even a free-standing PCBN blank, in a refractorymetal container. The container is sealed and returned to the UHTP press,where the powder is sintered together and to the substrate to form aPCBN tool blank. The PCBN tool blank is then either ground, lapped, wireEDM cut, or laser cut to shape and size, depending upon the application.

[0078] Superabrasives are materials that are defined as being processedunder high temperature and ultra high pressure. Superabrasive materialsinclude PCBN and polycrystalline diamond (PCD). These materials aregoing to be found on the periodic table and identified as compoundsincluding elements extending from IIIA, IVA, VA, VIA, IIIB, IVB, and VB.

[0079] Superabrasives have a hard primary or first phase, and asecondary catalytic phase that facilitates primary phase crystalstructure sintering and transformation. Superabrasives may or may not beelectrically conductive. They can also be strengthened using whiskerreinforcement. They may also be considered as materials that undergo asolid-state phase transformation during processing at elevatedtemperature and pressure, and a material that is created by a sinteringprocess, with or without a binder.

[0080] Another aspect of the invention concerns the shoulder 32.Depending upon how it is manufactured, the superabrasive material on theshoulder 32 may be relatively thin. This becomes important if thesuperabrasive material is being finished to a desired form. If thefinished form includes a slanted, beveled or angled surface or othersimilar structure as shown in this embodiment, it is important that theslant not pierce the superabrasive material. Accordingly, the thicknessof the superabrasive must be sufficient to provide the desired slantwithout reaching the substrate material.

[0081] As the present invention was being developed, a tool designed forfriction stir welding of aluminum was used on a ferrous alloy. The toolfailed at the pin. This is because there are geometrical considerationsthat must be taken into account when friction stir welding hardermaterials with higher melting points, and when using a shoulder and pinthat are coated with a superabrasive material. Thus, other novelfeatures of the invention include 1) the elimination of stress risers,2) the use of larger radii or chamfers, 3) more uniform distribution ofstresses, and 4) increasing the diameter of the pin.

[0082] Regarding stress risers, many prior art patents teach exotic pindesigns, including threaded pins, and pins having sharp edges andangles. Screw threads on a pin are generally desirable because thethreads push the workpiece material back down into the workpiece causinga stirring action and a better weld. However, these shapes are generallyundesirable in the present invention because they function as crackinitiators for a superabrasive coating, but can be used in a modifiedform to minimize the stress riser. Therefore, large radii or chamfers onthe shoulder 32 and the pin 34 are desirable. These large radii areshown in FIG. 2A.

[0083] Regarding pin diameter, the pin 34 of the preferred embodiment islarger in diameter than conventional tools. This is partly due to thegreater stresses that the pin 34 will experience when friction stirwelding MMCs, ferrous alloys, and superalloys. The pin diameter isprobably best expressed as a ratio of pin diameter compared to pinlength. In the presently preferred embodiment, the range of ratiosextends from 0.2:1 to 30:1.

[0084] It is also noted that the shoulder 32 is not shown as a flatsurface relative to a workpiece. The shoulder 32 is in fact concave.This shape enables the plasticized workpiece material to be more easilydisplaced and flow around the pin 34. The concave shape also forces theplasticized workpiece material back into the workpiece.

[0085] Although a relatively flat region 44 is shown between the outerand inner radii of the shoulder 32, this region 44 can also be curved toform a concave or a convex surface. Alternatively, the shoulder 32 canbe also be convex or flat relative to the workpiece.

[0086] The friction stir welding process requires that the tool holderpress down on the tool 28. This axial pressure is generally sufficientto hold the components 30, 32, 34 together in the axial direction.However, as the tool 28 is translationally moved with respect to theworkpiece, significant rotational forces are urging the shank 30 to moverelative to the shoulder 32. It is critical that the elements notturn-relative to each other. Relative rotational movement would preventthe pin from generating sufficient frictional heat against the weldregion of the workpiece.

[0087] Therefore, it is a novel element of the invention to require thatthe elements be mechanically locked. Mechanical locking is necessarybecause brazing the components together will only serve to function as apoint of weakness of the tool 28. This is because the brazing materialis likely to have a melting point that is at most near the temperatureat which friction stir welding is being performed.

[0088] Mechanical locking was first accomplished via dovetailing inearly experiments. However, the dovetails propagated crack formation inportions of the tool 28. Therefore, it is preferred to use flats 46 asshown in FIG. 2B. The flats 46 prevent slipping of the tool 28components 30, 32, 34 relative to each other, combination with a lockingcollar. Although the figure shows two flats, 1 to any desired number canbe disposed around the circumference of the tool 28.

[0089] Alternatively, the flats 46 can be replaced by other surfacefeatures that enable the tool components 30, 32, 34 to be mechanicallylocked into a position where they will remain stationary relative toeach other. The other surface features include the use of splines,friction welding, diffusion welding, a lock on back, or a lock on theoutside diameter of the shank.

[0090] The final components of this preferred embodiment are the collar36 and the thermal flow barrier 40. One of first collar materials thatwas used in experimentation was formed of a titanium alloy.Disadvantageously, titanium alloy is drawable, and will creep and flowunder high temperatures. Initial tests with a titanium collar showedthat the titanium alloy collar actually fell down around the shoulderand pin and onto the workpiece as the tool made a welding run.

[0091] Accordingly, it was decided that another material would befastened around the shoulder 32 and the shank 30 to mechanically lockthem together. In addition, the titanium alloy would still be present inorder to insulate the new collar material from the high temperatures ofthe shoulder 32 and pin 34. This insulation also assisted in thermalmanagement to thereby maintain the desired temperature at the weldingregion of the workpiece. The presently preferred embodiment utilizes asuperalloy for the material of the collar 36. For example,nickel-cobalt, or cobalt-chromium are suitable superalloy materials.

[0092]FIG. 2B is provided as an end-view of the tool 28. The materialsthat are visible from this perspective are the pin 34, the shoulder 32,the titanium alloy thermal flow barrier 40, and the collar 36.

[0093] Although the preferred embodiment teaches the use of CBN as thesuperabrasive coating on the shoulder 32 and the pin 34, this is not theonly superabrasive material that can be used. For example, one of thebest substitutes for CBN is polycrystalline diamond. It is well knownthat PCD exhibits many of the performance characteristics of CBN.

[0094] Dimensions of the preferred embodiment are only useful as anexample, but will be provided. The diameter of the pin is 0.37″. Thediameter of the shoulder is 1″. The thickness of the titanium alloythermal barriers 38, 40 are 0.060″, and the diameter of the collar 36 is1.63″. The angle on the collar 36 is shown as 15 degrees, and the angleof the shoulder is shown as 6 degrees. These figures are forillustration purposes only, and should not be considered limiting. Norwill these dimensions work for all applications.

[0095] There are various issues that need to be explained in order tounderstand all of the advantages and requirements of the presentinvention, and how the preferred embodiment was developed.

[0096] One important consideration of the tool 28 is that while CBN is agood material for friction stir welding steel, it may not be good forother materials. Therefore, it is an element of the invention to make itpossible to mix and match shoulders and pins, as will be shown inalternative embodiments.

[0097] Another consideration is that some superabrasives are soluble incertain materials. For example, PCD has a chemical reaction with atitanium alloy at friction stir welding temperatures. Thus, diamondcannot be used to weld materials that are carbide formers, unless thehighest temperature that will be reaches is below a soluble point.

[0098] There are two distinct advantages to using superabrasives in theshoulder 32 and the pin 34 of the present invention. First of all, thecoefficient of friction of CBN and of diamond is very low (0.05 to 0.1).In contrast, the coefficient of friction of steel is 0.8. This lowcoefficient of friction enables the workpiece material to slide alongthe tool 28 instead of sticking to it. The result is a much cleanerfinish that does not require a lot of finishing work. Finishing costscan be high, especially with ferrous alloys and superalloys. The lowcoefficient of friction also leads to reduced tool wear.

[0099] Second, the thermal conductivity of CBN and PCD are high, about100 to 550 Watts/meter-K, compared to steel which is about 48Watts/meter-K. The result is that the weld is cooler. Cooler welds aredesirable because they form further away from the melting point, andthus avoid all of the problems of liquid welding phases. It has beendemonstrated in tests that one of the direct benefits of the presentinvention is that the welds have greater tensile strength compared towelds using more conventional arc welding. Of course, the high thermalconductivity of CBN is also the reason for the user of thermal flowbarriers 38, 40 that are used to keep the heat from escaping the weldregion of the workpiece.

[0100] It has been explained that a substrate for the shoulder and thepin has been coated with a superabrasive. It is another novel element ofthe invention to allow for multiple coatings. These coatings can beapplied using CVD, ion-implantation, or PVD processes. The purpose ofthe coatings is to provide features that will assist the superabrasiveto withstand the different types of wear that it experiences. Forexample, a second coating can enhance the chemical wear resistance.

[0101] The coatings that are applied to substrates or on top of thesuperabrasives can be of varying thicknesses. Although in the abrasivetool industry a coating of 0.030″ to 0.050″ is considered a thickcoating, and a coating of less than 0.001 is considered a thin coating,it is an aspect of the invention that other thicknesses may be requiredfor optimum performance of the coating material. Solid CBN can also bepressed so that it has no coating. This CBN can be pressed to as large avolume as the UHTP process will allow, usually up to 4 inches indiameter by 4 inches long. This solid CBN does not, however, have thebenefit of a substrate that adds strength and toughness.

[0102] While thermal management is a novel element of the preferredembodiment, cooling of the tool is also important, but for a differentreason. Thermal management is used to ensure that enough heat isdirected to the weld region by making sure it is not siphoned away. Butfor the heat that is able to move away from the shoulder and pin, it isoften necessary to provide some type of active cooling. Cooling can takethe form of a mist directed at the exterior of the tool, or even air.But cooling can also be an internal process. Thus, it may be necessarywith some materials to provide internal cooling by providing a coolingchannel through a portion of the shank. It is also possible to cool thetool holder. Cooling can even extend to the workpiece itself. While heatis necessary for the weld, it should always be remembered that a coolweld is inherently stronger, and that friction stir welding is asolid-state process.

[0103] There are several alternative embodiments that must also beconsidered in order to understand performance issues. The presentlypreferred embodiment teaches a tool having two components, the shank 30,and the shoulder 32 with an integral pin 34. However, experimentationhas shown that the pin 34 will usually wear out before the shoulder 32.If they are integral, the entire shoulder 32 and pin 34 combination haveto be replaced together. This is a waste of resources.

[0104] Therefore, FIG. 3 is a cross-sectional perspective view of analternative embodiment wherein the shoulder 50 is not integral with thepin 52. Instead, these components are manufactured separately, andcoupled to the shank 48. As shown in FIG. 3, the pin 52 rests within abore hole 54 drilled into an end of the shank 48. The thermal flowbarriers 38, 40 are still in place, except for where the pin 52 extendsinto the shank 48. However, if the pin 52 only has a superabrasivecoating on the portion that is outside of the bore hole 54, then thecemented tungsten carbide (or other suitable) substrate of the pin 52 isnot going to be as good a thermal conductor to the shank 48.Nevertheless, in an alternative embodiment, the thermal barrier can beextended down into the bore hole 54.

[0105] Coupling the pin 52 to the shank 48 is also not a trivial matter.Preferably, the pin 52 is disposed into the bore hole 54 using a pressfitting. However, it is likely that a hex or square shaped hole may bedesirable. It is noted that it is possible to add strength to the pin 52if it is put into residual compressive stress.

[0106] Residual compressive strength can be created, for example, byheating the tool. As the tool is heated, it expands. The diameter of thepin is selected so that when the tool cools, it exerts positivemechanical pressure on the pin.

[0107] Another method of attachment might be to dispose a screw into thepin 52. The screw would be used to pull the pin into compression throughthe back end of a tool.

[0108]FIG. 4 is provided as another alternative embodiment of thepresent invention. The difference from FIG. 3 is that the hole 62 nowextends entirely through the shank 60. One of the main advantages ofthis embodiment is that replacing the pin 64 is simply a matter ofpushing the pin 64 out of the shank 60 by inserting a tool through thehole 62. This design can reduce costs, and make the tool reusable formany applications. It is thus only necessary to insert a pin 64 of theproper length.

[0109] One aspect of the invention that has not been addressed is thecomposition of the pin when it is a replaceable item. Preferably, thepin is manufactured from cemented tungsten carbide, and coated with anappropriate superabrasive. However, the pin can also be manufactured asa solid superabrasive material, or be a carbide with the desiredcoating.

[0110] Another aspect of the pin is that it can be reinforced.Reinforcing a pin may be desirable if the pin length is unusually longbecause of the thickness of the workpiece material. Reinforcement mayalso be necessary when the material of the pin does not inherently havethe strength of a material such as tungsten carbide.

[0111] In a similar manner, the shank can also be manufactured from asuperabrasive, or be a carbide that is coated with a superabrasive.

[0112]FIG. 5 is another alternative embodiment of the invention, whereininstead of having separate components, the tool 70 is a monolithic unit.However, the cost of manufacturing an entire tool as a single piece isprohibitively expensive. Given the advantages of the other embodiments,it is unlikely that this embodiment will be widely used. Nevertheless,it is an option that would likely be formed from cemented tungstencarbide, with a superabrasive coating applied to the shoulder 72 and pin74 areas. The difficulty in its use might be the thermal management thatis seen as critical when using superabrasives. Therefore, insertion of athermal barrier may be important, but that would defeat the purpose ofthe monolithic design.

[0113] An alternative embodiment of the invention is the type of pinthat is inserted into the tool. FIG. 6A is provided as a profileperspective view of a helical endmill blank 80. The substrate of theblank 80 is preferably cemented tungsten carbide, with the PCBN or othersuperabrasive disposed in helical channels 82. FIG. 6B is an end view ofthe blank 80, illustrating the helical channels 82 in which thesuperabrasive material is disposed.

[0114] It is envisioned that there will be other means for coupling aPCBN shoulder or coated shoulder to a shank. FIG. 7A is provided as across-sectional illustration of another tool embodiment that providesall of the desirable characteristics of the present invention, butwithout the use of a separate locking collar. In this figure, thelocking collar is replaced by a portion of the shank itself so that itis integral to the shank itself.

[0115] Specifically, the shank 90 is shown having a bore hole 92disposed partially into the working end 94 of the shank. The depth ofthe bore hole 92 is selected based upon the depth of the shoulder andthe pin 96. In this embodiment, the shoulder and the pin 96 areintegral. However, the shoulder and the pin 96 could also be separatecomponents as shown in previous embodiments. What is important aboutthis embodiment is that the wall 98 around the bore hole 92 functions asa locking collar, to thereby assist in preventing rotational movement ofthe shoulder and the pin 96 relative to the shank 90. This can beaccomplished, for example, by press fitting the shoulder and the pin 96into the bore hole 92. Notice that the thermal flow barrier 100 is alsoin place to enable management of heat from the shoulder and the pin 96to the shank 90.

[0116] However, it is possible that under some load conditions, theshoulder and the pin 96 may slip despite the press fitting. Therefore,it is also envisioned that this embodiment includes the use of someother means for mechanically locking the back surface 102 of theshoulder and the pin 96 to the shank 90. This can be accomplished usingsome of the previously mentioned techniques. For example, mechanicallocking can be performed by complementary dentations, splines or otherphysical features on the back surface 102 and the bottom surface 104 ofthe bore hole 92 that prevent relative rotational movement throughcomplementary interlocking.

[0117]FIGS. 7B and 7C are provided as an illustration of just twoexamples of how the back surface 102 of the shoulder and the pin 96 canbe mechanically locked to the bottom surface 104 of the bore hole 92 ofthe shank 90. In this figure, bore hole splines 106 are formed in thebottom surface 104 of the bore hole 92, and complementary splines 108are formed on the back surface 102 of the shoulder and the pin 96.

[0118] An important and novel aspect of the invention also pertains tothe flow of the workpiece material around the tool pin. Althoughfriction stir welding is said to occur as a solid-phase process, theworkpiece material is still capable of fluid-like movement, or flow. Itis important when trying to obtain the best weld possible to increasethe rate of flow of the workpiece material around the pin.

[0119] Laminar flow is defined as non-turbulent fluid flow.Unfortunately, laminar flow of the workpiece material is also theslowest. Therefore, any geometry of the pin that will result in theincreased rate of fluid flow of the workpiece material around the pinwill also result in a weld having the improved weld characteristics.Accordingly, it is desired to have a turbulent fluid flow of theworkpiece material, or transitional flow which is defined as a flow thathas turbulent characteristics. Therefore, it is desirable to trip aboundary layer from laminar flow into the transitional or turbulent typeof flow. Furthermore, it is also desirable to obtain the transitional orturbulent flow at the lowest possible rotational speeds of the tool, andwith the simplest tool geometry.

[0120] It is therefore an aspect of the invention to teach a pingeometry that will result in transitional or turbulent flow of theworkpiece material. FIG. 8 is provided as a profile view of a pin havingphysical deformations that are designed to obtain at least sometransitional or turbulent flow of the workpiece material around the pin.As shown, the pin 120 is covered by a plurality of dimples 122, muchlike the dimples of a golf ball. The number, size, and depth of thedimples 122 will need to be varied in order to obtain the desired flowcharacteristics for the workpiece material.

[0121] Similarly, FIG. 9A is provided as an end view of a pin 126 andshoulder 128 that is designed to generate transitional or turbulent flowaround the pin. The pin 126 is shown having a single flat 130 on a sidethereof. It is envisioned that the total number and the width of theflats 130 can be adjusted to obtain the desired flow characteristics ofthe workpiece. It is also envisioned that instead of a flat surface, thesurface irregularity will extend the length of the pin, and may not be auniform surface. For example, FIG. 9B shows another end view of a pin132 and shoulder 134, where a surface irregularity 136 is not flat.

[0122] Another aspect of the invention also related to obtainingtransitional or turbulent flow around the pin is shown in FIG. 10. FIG.10 is a profile view of a tool 140, where the pin 142 is disposedparallel to but no longer concentric with a lengthwise axis 144 of thetool. The pin 142 is now offset, thereby creating a cam configurationthat is designed to generate transitional and turbulent flow in theworkpiece material. It is noted that the degree of offset is exaggeratedfor illustration purposes only. The actual offset will depend upon thetool and the workpiece characteristics.

[0123] It is also envisioned that many useful pin geometries and toolscan be adapted in accordance with the principles of the presentinvention. For example, tools having pins of adjustable length canprovide many benefits. The tools must be modified to reduce stressrisers, either coated on the shoulder and pin with superabrasivematerials or manufactured from solid superabrasive materials, andutilize thermal management techniques as taught in the presentinvention.

[0124] A last aspect of the invention is the subject of pressing a toolto a near net shape. Near net refers to a tool that after pressingrequires very little finishing to obtain the final product. In thepresently preferred embodiment, the pin, shoulder, integral pin andshoulder, and pin with reinforcement are pressed to near net shape.

[0125] It is to be understood that the above-described arrangements areonly illustrative of the application of the principles of the presentinvention. Numerous modifications and alternative arrangements may bedevised by those skilled in the art without departing from the spiritand scope of the present invention. The appended claims are intended tocover such modifications and arrangements.

What is claimed is:
 1. A friction stir welding tool that is capable offunctionally friction stir welding metal matrix composites (MMCs),ferrous alloys, non-ferrous alloys, and superalloys, said friction stirwelding tool comprising: a friction stir welding tool having a shank, ashoulder and a pin, wherein the shoulder is mechanically locked to theshank to thereby prevent rotational movement of the shoulder relative tothe shank; and a superabrasive material disposed on at least a portionof the shoulder and the pin, wherein the superabrasive material has afirst phase and a secondary phase, wherein the superabrasive material ismanufactured under an ultra high temperature and an ultra high pressureprocess; and wherein the friction stir welding tool is capable offunctionally friction stir welding MMCs, ferrous alloys, non-ferrousalloys, and superalloys.
 2. The tool as defined in claim 1 wherein thetool further comprises the superabrasive being selected from the groupof materials comprised of compounds including elements extending fromIIIA, IVA, VA, VIA, IIIB, IVB, and VB on the periodic table of theelements.
 3. The tool as defined in claim 2 wherein the tool furthercomprises selecting polycrystalline cubic boron nitride (PCBN) orpolycrystalline diamond (PCD) as the superabrasive material.
 4. The toolas defined in claim 2 wherein the tool further comprises utilizing awhisker reinforced superabrasive material to thereby inhibit spalling ofthe superabrasive material.
 5. The tool as defined in claim 2 whereinthe tool further comprises selecting the superabrasive material in orderto obtain a desired balance between chemical wear and mechanical wear ofthe friction stir welding tool.
 6. The tool as defined in claim 2wherein the tool further comprises the shank, wherein a material for theshank is selected from the group of materials comprised of cementedtungsten carbide, steels, and superabrasive materials.
 7. The tool asdefined in claim 2 wherein the tool further comprises an internalcooling system for cooling the friction stir welding tool.
 8. The toolas defined in claim 1 wherein the tool further comprises at least asecond material disposed over the superabrasive coating to therebycontribute properties of the second material to the superabrasivecoating.
 9. The tool as defined in claim 2 wherein the tool furthercomprises a first thermal flow barrier disposed between the shoulder andthe shank to thereby regulate movement of heat from the shoulder to theshank.
 10. The tool as defined in claim 9 wherein the tool furthercomprises a locking collar, the locking collar performing the functionof mechanically locking the shoulder to the shank to thereby preventrotational movement of the shoulder relative to the shank.
 11. The toolas defined in claim 10 wherein the tool further comprises a secondthermal flow barrier disposed between the locking collar and the portionof the shoulder and the shank around which it is disposed, to therebyregulate movement of heat from the shoulder and the shank to the lockingcollar.
 12. The tool as defined in claim 11 wherein the first thermalflow barrier and the second thermal flow barrier further comprisestitanium alloys.
 13. The tool as defined in claim 12 wherein the shankfurther comprises a material that is selected having a thermalconductivity that is less than the shoulder, the pin and the lockingcollar.
 14. The tool as defined in claim 13 wherein the shank isselected from the group of materials comprised of cemented tungstencarbide, tungsten alloys, steels, molybdenum alloys, and superalloys.15. The tool as defined in claim 14 wherein the tool further comprisesproviding at least one surface feature disposed along a lengthwise axisof the tool, wherein the surface feature enables the locking collar tomore securely restrain the shoulder and the shank in a same relativeposition.
 16. The tool as defined in claim 15 wherein the tool furthercomprises selecting the at least one surface feature from the group ofsurface features comprising a flat, a spline, a keyway and key, alocking pin, a dovetail, and a dentation.
 17. The tool as defined inclaim 16 wherein the tool further comprises a mechanical lock betweenthe shank working end and the shoulder attaching end, the mechanicallock being selected from the group of mechanical locks comprised ofdovetails, splines, and dentations.
 18. The tool as defined in claim 17wherein the shoulder further comprises a shoulder radii disposed about aworking edge thereof, the shoulder radii functioning as a crackinhibitor in the superabrasive material.
 19. The tool as defined inclaim 18 wherein the means for mechanically locking the shoulder to theshank is selected from the group of mechanical locking means comprisedof splines, locking pins, dovetails, and dentations.
 20. The tool asdefined in claim 19 wherein the tool further comprises the lockingcollar, wherein the material selected for the locking collar is asuperalloy.
 21. The tool as defined in claim 20 wherein the tool furthercomprises the locking collar, wherein a material selected for thelocking collar is selected from the group of materials comprised ofnickel-cobalt and cobalt-chromium.
 22. A friction stir welding tool thatis capable of friction stir welding metal matrix composites (MMCs),ferrous alloys, non-ferrous alloys, and superalloys, said friction stirwelding tool being a monolithic device comprising: a shank having ashaft attaching end and a shaft working end; a shoulder formed on theshaft working end, the shoulder having a shoulder working edge, whereinthe shoulder working edge is formed as a radii; a pin formed in theshoulder, wherein the pin is concentric with and parallel to alengthwise axis of the shoulder from which it extends outwardly, andwherein a first pin radii is formed at a junction between the shoulderand the pin, and a second pin radii is formed at a pin working edge. asuperabrasive material disposed on at least a portion of the shoulderand the pin, and wherein the friction stir welding tool is capable offunctionally friction stir welding MMCs, ferrous alloys, non-ferrousalloys, and superalloys.
 23. A method for friction stir welding metalmatrix composites (MMCs), ferrous alloys, non-ferrous alloys, andsuperalloys, said method comprising the steps of: (1) providing afriction stir welding tool having a shank, a shoulder and a pin; (2)mechanically locking the shoulder to the shank to thereby preventrotational movement of the shoulder relative to the shank; and (3)disposing a superabrasive material on at least a portion of the shoulderand the pin, wherein the superabrasive material has a first phase and asecondary phase, wherein the superabrasive material is manufacturedunder an ultra high temperature and an ultra high pressure process, andwherein the friction stir welding tool is capable of functionallyfriction stir welding MMCs, ferrous alloys, non-ferrous alloys, andsuperalloys.
 24. The method as defined in claim 23 wherein the methodfurther comprises the step of selecting the superabrasive material fromthe group of materials comprised of compounds including elementsextending from IIIA, IVA, VA, VIA, IIIB, IVB, and VB on the periodictable of the elements.
 25. The method as defined in claim 24 wherein themethod further comprises the step of selecting polycrystalline cubicboron nitride (PCBN) or polycrystalline diamond (PCD) as thesuperabrasive material.
 26. The method as defined in claim 25 whereinthe method further comprises the step of inhibiting spalling of thesuperabrasive material by utilizing a whisker reinforced superabrasivematerial.
 27. The method as defined in claim 26 wherein the methodfurther comprises the step of achieving a balance between chemical wearand mechanical wear of the friction stir welding tool by selecting thesuperabrasive material having a first percentage of first phasematerial, and having a second percentage of second phase material. 28.The method as defined in claim 27 wherein the method further comprisesthe step of selecting a hardened material for the shank, wherein theshank material is selected from the group of materials comprised ofcemented tungsten carbide, steels, and superabrasive materials.
 29. Themethod as defined in claim 24 wherein the method further comprises thestep of disposing at least a second material over the superabrasivecoating to thereby contribute properties of the second material to thesuperabrasive coating.
 30. The method as defined in claim 29 wherein themethod further comprises the step of applying the at least a secondmaterial over the superabrasive coating selecting an application methodfrom the group of application methods comprised of CVD, ion-beamimplantation, and PVD.
 31. The method as defined in claim 30 wherein themethod further comprises the step of regulating movement of heat betweenthe shoulder and the shank by disposing a first thermal flow barrierbetween the shoulder and the shank.
 32. The method as defined in claim31 wherein the method further comprises the step of preventingrotational movement of the shoulder relative to the shank by providing alocking collar to mechanically lock the shoulder to the shank.
 33. Themethod as defined in claim 32 wherein the method further comprises thestep of regulating movement of heat from the shoulder and the shank tothe locking collar by providing a second thermal flow barrier betweenthe locking collar and the portion of the shoulder and the shank aroundwhich it is disposed.
 34. The method as defined in claim 33 wherein themethod further comprises the step of providing titanium alloys as thefirst thermal flow barrier and the second thermal flow barrier.
 35. Themethod as defined in claim 34 wherein the method further comprises thestep of regulating movement of heat within the friction stir tool byselecting a material for the shank that has a lower thermal conductivitythan is less than the shoulder, the pin and the locking collar.
 36. Themethod as defined in claim 35 wherein the method further comprises thestep of selecting the material for the shank from the group of materialscomprised of cemented tungsten carbide, tungsten alloys, steels,molybdenum alloys, and superalloys.
 37. The method as defined in claim36 wherein the method further comprises the step of inhibiting crackpropagation in the superabrasive material on the shoulder by providing ashoulder radii disposed about a working edge thereof.
 38. The method asdefined in claim 37 wherein the method further comprises the step ofselecting a superalloy for the locking collar to thereby preventrotational movement of the shoulder relative to the shank.
 39. Themethod as defined in claim 38 wherein the method further comprises thestep of selecting a superalloy for the locking collar from the group ofsuperalloys comprised of nickel-cobalt and cobalt-chromium.
 40. Themethod as defined in claim 24 wherein the method further comprises thestep of reducing stress risers on the shoulder and on the pin, tothereby inhibit crack propagation of the superabrasive material.
 41. Themethod as defined in claim 40 wherein the method further comprises thesteps of: (1) forming the shank as a generally cylindrical object; and(2) providing the shoulder as a disk-like object, wherein the pin is anintegral component of the shoulder, wherein the pin is generallycylindrical, and wherein the pin is concentric with and parallel to alengthwise axis of the shoulder from which it extends.
 42. The method asdefined in claim 41 wherein the method further comprises the step ofproviding a locking collar, the locking collar mechanically locking theshoulder to the shank to thereby prevent rotational movement of theshoulder relative to the shank.
 43. The method as defined in claim 24wherein the method further comprises the step of making an improvedfriction stir weld by providing a friction stir welding tool thatinhibits materials from adhering to the friction stir welding toolduring the welding process by utilizing a superabrasive material havinga low coefficient of friction.
 44. The method as defined in claim 24wherein the method further comprises the step of regulating a pindiameter to pin length ratio to thereby control characteristics of aweld.
 45. The method as defined in claim 24 wherein the method furthercomprises the steps of: (1) providing a shank having a shaft working endand a shaft attaching end, wherein a shank bore hole is disposed fromthe shaft working end to the shaft attaching end, and wherein the shankbore hole is concentric with a lengthwise axis; (2) providing a shoulderhaving the form of a disk, wherein a shoulder hole is aligned with theshank bore hole, and wherein the shoulder is coupled to the shank,wherein the shoulder is mechanically locked to the shank, therebypreventing rotation of the shoulder relative to the shank; and (3)providing a pin disposed through the shoulder hole and at leastpartially into the shank bore hole, wherein a portion of the pin isdisposed outside the shoulder hole, and wherein the pin is mechanicallylocked to the shank, thereby preventing movement rotation of the pinrelative to the shank.
 46. The method as defined in claim 24 wherein themethod further comprises the step of increasing a rate of flow ofmaterial around the pin during a friction stir welding process tothereby improve characteristics of a weld.
 47. The method as defined inclaim 46 wherein the method further comprises the step of creatingtransitional flow or turbulent flow of material around the pin.
 48. Themethod as defined in claim 24 wherein the method further comprises thestep of reducing wear on the friction stir welding tool by utilizing asuperabrasive material having a low coefficient of friction.
 49. Themethod as defined in claim 24 wherein the method of friction stirwelding MMCs, ferrous alloys, non-ferrous alloys, and superalloysfurther comprises the step of utilizing thermal management to therebyregulate thermal wear of the friction stir welding tool.
 50. The methodas defined in claim 49 wherein the method further comprises the step ofutilizing superabrasives to thereby reduce mechanical wear and chemicalwear of the friction stir welding tool.
 51. The method as defined inclaim 51 wherein the method further comprises the step of providingthermal flow barriers to regulate movement of heat within the frictionstir welding tool.
 52. The method as defined in claim 51 wherein themethod further comprises the step of controlling geometry of thefriction stir welding tool to enable use of superabrasives.